The invention relates to a device for producing a rotatable, but axially fixed connection between a rotary axle of a chain guide of a rear gear change mechanism for bicycles and a related lower pivot head (i.e., a derailleur front knuckle).
Rear gear change mechanisms on bicycles serve to shift the chain between several sprockets mounted on the rear wheel. To this end, these rear gear change mechanisms include a stationary part, a moveable part and a parallelogram mechanism. The stationary part is mounted on the bicycle frame, usually on a rear dropout, and includes, besides an element like a screw for mounting the gear change mechanism on the dropout, an upper pivot head. The essential component of the moveable part is a lower pivot head, which moves jointly with a chain guide. In addition to the function of guiding the chain, the chain guide must make it possible to tension the chain. To this end, the chain guide is mounted on the lower pivot head so that this chain guide can rotate relative to said lower pivot head.
In some cases the corresponding axle is rigidly connected to the chain guide and is accommodated in a borehole on the pivot head. Current designs show a screw connection between the axle and the chain guide, such as in DE 32 35 138. DE 36 23 887 shows how a spring plate engages with a circumferential groove on the axle and secures the axle in its axial position so as to be rotatable relative to the pivot head. In this solution the passage on the lower pivot head, which is provided for mounting the spring plate, is still present following assembly, as a result of which it is still possible for dirt and moisture to penetrate into the interior.
The result is the possibility of a malfunction. If the spring plate is also accessible from the direction opposite the mounting direction, then it can also be disassembled again by exerting a compressive force. Instead of the spring plate, it is also possible to use a pin that is pressed transversely to the axle into a borehole in the lower pivot head. This pin also engages with a circumferential groove on the pivot head, but, in contrast to the solution with a spring plate, has the drawback that an engagement with the axle in the groove is produced only at smaller contact areas, a state that makes the arrangement more sensitive to wear. In addition, the pin cannot be disassembled again or can be disassembled only with great difficulty.